This invention relates to radiotelephone systems for serving a plurality of remote subscriber stations and, more particularly, to a radiotelephone system in which certain of said subscriber stations are located in a physically adjacent group.
A radiotelephone system including a base station for serving remote subscriber stations is described in U.S. Pat. No. 5,119,375. In that system each subscriber station was equipped with a radio that could be instructed by the base station to tune to a particular channel and to employ a particular time slot for the duration of a given conversation. Time division multiplex (TDM) radio channel transmission was employed from the base station to the subscriber stations and time division multiple access (TDMA) transmission from the individual subscriber stations to the base station. The time division of each radio channel into time slots and the compression of speech signals permitted each radio frequency channel to support a number of voice paths equal to the number of time slots. Analog voice signals to and from the public switched telephone network were first converted to 64 kbps xcexc-law companded pulse coded modulation (PCM) digital samples. Before transmission over the radio channel the-digital samples were subjected to voice-compression to reduce the voice information rate from 64 kbps to 14.6 kbps using residual excited linear predictive (RELP) coding. A voice codec and modem were required to be dedicated to a specific frequency and time slot for the duration of a call.
While the foregoing system operated in a highly satisfactory manner in allowing telephone service to be provided especially to areas where wire lines are impractical, the unforeseen growth of such telephone service has given rise to situations in which several subscriber stations are found to lie in close proximity with one another. Initial efforts to lower the per-line cost of serving a group of such closely situated subscriber stations were focused on consolidating the installation and maintenance costs of individual subscriber stations through the sharing of common equipment such as the enclosure, power supply, RF power amplifier and antenna. Thus, in a closely situated group of subscriber stations, each of which could access an RF channel, a single broadband RF power amplifier could be employed to serve the group. However such efforts still required each subscriber line to have its own modem and radio transceiver. The individual transceiver outputs were fed to the common RF power amplifier, which had to be designed to handle a peak power equal to the sum of the power of all of the transceivers in the group of adjacent subscriber stations that could simultaneously be active on the same time slot. It is apparent that further consolidation over that possible in the ""375 patent system and a reduction in the peak and average power required would be desirable, especially in remote areas required to be served by solar cell power.
In accordance with the principles of our invention, per-line costs are reduced for a physically adjacent group of subscriber lines by permitting the lines within such a group to share not only a common power supply and RF power amplifier, but modem, synchronization, IF, up- and down-conversion and controller functions as well, so that significant concentration is achieved In our system, a small number of modems is provided to serve the multiple subscribers in a physically adjacent group, hereinafter referred to as cluster or, more particularly, as a modular cluster. In an illustrative embodiment, subscriber line circuits and modems are modularized printed circuit cards which plug into a frame employing backplane wiring to distribute timing information. and data among the units. Any of the modems may be seized to handle a call for any of the subscribers and each modem may handle calls for several subscribers on successive time slots. The same or a different frequency may be used to support communications for each subscriber on successive time slots.
It is a feature of our invention that the selection from the common pool of frequency-agile modems of the modem to be used to handle a call is controlled to conserve power consumption in two ways. First, a new modem is preferably not seized for use to handle a call until all of the time slots on active modems have been assigned to calls, thereby allowing all not-yet-selected modems to remain in a power-conserving, xe2x80x9cpowered-downxe2x80x9d state. Second, the number of calls using the same time slot (on different frequencies) is controlled to reduce the peak power demand on the RF power amplifier.
It is a further feature of our invention to avoid synchronization delay when it is necessary to seize a powered-down modem for use on a call. Once time slot synchronization with the base station has been established for the first modem of the pool at the cluster, synchronization information is made available to the remaining modems, advantageously over backplane wiring, under control of a microprocessor-based cluster controller. Accordingly, all powered-down modems remain instantly assignable to handle calls without undergoing any delay to become synchronized with the base station""s time division frame.
It is a further feature of our invention to classify modem synchronization states according to several synchronization parameters and to derive a confidence factor for each active modem that reflects the reliability of the synchronization parameters and to distribute synchronization information from the modem having the best confidence factor.